php web page unable to call embedded php frame in squirrelmail

Something about how I installed squirrelmail on my kubuntu 16.04 server is not right. After I log into squirrelmail, the webmail.php creates a display with two empty frames. If I browse to right_main.php (or left_main.php) those pages display correctly and the plugin scripts, such as /plugins/fetch_mail/fetch.php also work correctly. Unfortunately, sendmail, i.e. the compose.php script, also does not work.
I'm stumped after spending many hours trying to fix this. Tried installing roundcube as a workaround, but it doesn't work right either.